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Biofuel advance

Posted on 10 September 2010

Biofuel advance

Photo: David J. Tenenbaum

Gasoline already contains corn-ethanol; a new study shows a new way to make ethanol from switchgrass or waste wood.

Yeast can ferment corn starch into ethanol to be added to gasoline, but that diverts millions of tons of food from hungry people. Researchers are trying to ferment many other plant carbohydrates, especially cellulose, the tough chain-like molecule that stiffens the cell wall so plants can stand by themselves.
Unfortunately, the yeasts used to make ethanol have no taste for cellulose.
In this week’s Science, Jamie Cate, in the department of molecular and cell biology at the University of California at Berkeley, reports a transfer of two genes from a fungus to ethanol-making yeast. Although the fungus was discovered on French bread in the 1840s, the result was not exactly a fine Burgundy, or even a gallon of cheap jug wine, but it was a proof of principle that a single organism could, almost single-yeastedly, convert cellulose into ethanol.
Mon dieu!
The advance may hasten the day when waste wood, crop residues and fast-growing crops such as switchgrass can replace edible crops like corn and sugar cane in producing fuel.

Woody biomass or wood waste could be made into biofuel for cars, trucks or airplanes.

Raise a glass to success!

“It’s a proof of principle using lab strains,” says Cate.
The genetic transfer enabled a single strain of yeast to convert cellulose in plant cell walls into ethanol. After commercial enzymes busted the cellulose into short chains of glucose units, the yeast:

Transported those chains inside the yeast cell,

Converted the chains into individual glucose molecules, and

Fermented that glucose to ethanol (which is what the yeast does naturally).

If biofuels can be made from plant material, the net global warming impact should be zero, since growing plants absorb carbon dioxide from the atmosphere.

The short chains of glucose that the Neurospora crassa fungus extracts from cellulose do not normally enter the yeast cell, but the transporters ensure that they will enter the transformed yeast, enabling the yeast to make ethanol from normally indigestible compounds.

Taking lessons from fungi

The research began with a basic question. A large portion of plant biomass is cellulose, and “microorganisms in the wild live on plants; they obviously have figured out how to degrade plants as food,” says Cate. “Plants have been figuring out ways to prevent microbes from doing this, so there’s this ongoing battle, and we knew some fungi would be very good at decomposing cellulose.”
Cate focused on N. crassa, a well-studied fungus that lives in burned-over areas, but also has a taste for a stale baguette. The research team moved two genes from the fungus into Saccharomyces cerevisiae, a yeast widely used to ferment sugar into ethanol.
One gene forms structures in the yeast’s cell wall that draw short chains of glucose into the cell. The second gene makes beta-glucosidase, an enzyme that the fungus (and now the yeast) use inside the cell to snip the short chains of glucose into individual glucose molecules, where the yeast converts them into ethanol.

Switchgrass has less environmental impact than corn, and so may be a better source of ethanol. But switchgrass plantations could still divert land needed to grow food.

Raise a glass to success!

Although the short chains of glucose that the fungus extracts from cellulose is not digestible to normal yeast, the transformed yeast used these short chains to produce an abundance of ethanol. “It’s a proof of principle using lab strains,” says Cate. “We in the Energy Bioscience Institute [a collaboration of UC-Berkeley, the University of Illinois, Lawrence Berkeley Laboratory and BP] have colleagues who are helping us look at some really robust, industrial yeasts to see how the transporters work in those systems.”

Cellulose-eating yeast cells after transformation: The green marks the transporter structures made by genes moved from a cellulose-eating fungus.

Cate says transporters are key. “Any cell is a fortress, with a membrane or a cell wall that keeps things out to protect its innards. To get a small molecule in or out, there has to be a way, and these are the transporters, which live in the cell membrane, with parts on the outside and parts on the inside.”
The study was “a pretty slick example of how genomic technology can rapidly get you to the gene you care about,” says Steven Slater, associate director of the Great Lakes Bioenergy Research Center. “They used a combination of published literature on genes that are differentially expressed when several fungi are exposed to cellulose, and were able to rapidly go from there down to something that looks like transporter.”

Training a workhorse

The key, Slater says, is that “they took a workhorse organism that is primarily used for the production of ethanol and gave it a new genetic tool that could be used to get things other than glucose inside the cell; that’s important for producing ethanol from cellulosic biomass.”
Indeed, Cate says, many organisms have ways to transport fragments of cellulose: “You can find these all over in nature, including in the black truffle, a fungal delicacy that grows symbiotically on oak trees.”
Cate expects further progress. “We in the Energy Bioscience Institute [a collaboration of UC-Berkeley, the University of Illinois, Lawrence Berkeley Laboratory and BP] are testing some really robust, industrial yeasts.”
This process may not be limited to ethanol, Cate says. “It’s modular, and it may benefit research groups that have been working on yeast to make all sorts of interesting biofuels: alcohols, or things like diesel or jet fuel.”